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Pinholes in Coating

How can we prevent pinhole formation in bend areas of parts made from hot-rolled, pickled and oiled (HRPO) steel?

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Q. We powder coat parts made of approximately 1/8”-thick hot-rolled, pickled and oiled (HRPO) steel. The parts come out of the oven with a large number of pinholes through the coating in the outside radius of a bend area. This does not happen with thinner-gauge steel, but it happens a lot with the heavier material. We are using a five-stage washer with an iron phosphate conversion coating. Have you seen this before? Is there something we can do to prevent this? —K.P.

A. Yes, I have seen this before. When heavier-gauge steel is formed, it can create micro-fractures along the outside of the bend radius. Air is trapped in the pores of the fractured area, and it releases during cure, generating a series of small pinholes along the bend. If you can form the area in question with a larger radius it will help a lot. A tight radius will be more likely to fracture. It may also help to talk to your powder supplier about out-gassing protection in your powder formula. This may help provide a more forgiving flow cycle that will minimize the problem. If neither of these ideas solves the problem, you may be stuck with applying two coats of powder. The second coat should cover much better, as the trapped air has been relieved by the first cure cycle.

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